Barrage of Meteors May Have Doomed the Dinosaurs

By KENNETH CHANG

Published: March 11, 2003

Scientists are arguing again over the idea that the combination of cataclysms that doomed the dinosaurs 65 million years ago -- titanic volcanic eruptions in India and a meteor impact off the coast of Mexico -- may not have been a coincidence after all.

For decades, some geologists have theorized that the force of an extraterrestrial rock crashing into Earth could have cracked its crust thousands of miles away and allowed molten lava to spill out from the interior. But no one has yet found any solid evidence.

Now, though, researchers at University College London are suggesting that the Indian lava flows are the impact site of an earlier, larger meteor, and that evidence of the impact was submerged by upwelling lava. In this view, the mass extinction of dinosaurs and other creatures was caused not by a single meteor, but by a barrage of them.

The new work is provoking another burst of theories and debate over the demise of the dinosaurs, which has never been explained to everyone's agreement.

The new theory, which the researchers described in a scientific journal recently, holds that a meteor at least 12 miles wide -- at least twice as wide as the one that struck Mexico -- would melt some rock, but not nearly the amount seen in the lava flows, known as the Deccan Traps, which cover hundreds of thousands of square miles of what is now India.

Rather, the researchers said, the impact would cause ''decompression melting'' of already hot rocks deep within the earth. Tens of miles below the surface, temperatures reach more than 2,000 degrees Fahrenheit but rocks remain solid because of the high pressure exerted by the rocks weighing down above them.

Computer simulations indicate that once the meteor impact blew away the overlying rocks, the ones below, relieved of pressure, could then have turned to lava.

''The whole story is what happens underneath the crater,'' said Dr. Adrian P. Jones, a geologist at University College London and lead author of an article that appeared in Earth and Planetary Science Letters last year.

''It's rather like having a hot-air balloon and a pin. People have calculated the energy of the pin very accurately, but they've forgotten the balloon is going bang.''

This sequence may have played out several times in Earth's history. Notably, the largest of all mass extinctions 250 million years ago, at the Permian geological period and the beginning of the Triassic, coincided with the creation of lava flows known as the Siberian Traps, the largest of all of the volcanic eruptions.

There are also intriguing but ambiguous hints of a meteor impact at the Permian-Triassic boundary. Two years ago, a group of scientists reported finding buckyballs -- durable, soccer-ball-shaped carbon molecules -- that contained helium and argon gases with un-Earthlike chemical signatures.

The scientists said the buckyballs were molecular remnants of the meteor, but other researchers have been unable to verify the claim. Scientists have also found slightly elevated levels of iridium -- an element common in meteors -- in sediment layers dating to the Permian-Triassic boundary.

While the evidence for a connection any single event is sparse, Dr. Dallas H. Abbott of Columbia University's Lamont-Doherty Earth Observatory and Dr. Ann E. Isley of the State University of New York at Oswego say a compelling picture emerges when looked at over a longer view. They compiled evidence of meteor impacts and massive volcanic eruptions over most of Earth's history, dating back four billion years.

Skeptics like Dr. H. Jay Melosh, a professor of planetary sciences at the University of Arizona, are utterly unconvinced. ''I know it's a fun idea,'' he said. ''I think that's why so many people have been advocating it. It makes a good discussion after beer. But if you start looking at the details and the real evidence for this, it really falls apart.''

The dates of the ancient meteor impacts and eruptions in Dr. Abbott's and Dr. Isley's analysis can only be roughly estimated, within tens of millions of years, and the results depend on how the statistical analysis is performed. ''Some people get correlations, and some people don't,'' Dr. Melosh said.

Dr. Melosh also said that decompression melting cannot explain the Deccan Traps. While the meteor will punch deep into the Earth, the Earth will almost immediately rebound. ''There's a certain amount of willful misunderstanding here,'' he said.

Further, he said, there is no evidence anywhere on Earth that meteor impact has ever caused a volcanic eruption. And scientists still do not have a convincing model of how an impact could set off an eruption.

But Dr. Jones of University College London said there was evidence that decompression melting was a viable explanation. He cited Iceland, where, he said, the melting of glaciers has relieved enough pressure to accelerate eruptions there.

That is the latest in a multitude of theories that have tried to connect meteors and volcanoes.